Emergence of promising n-type thermoelectric material through conductive network and strong phonon softening

Abstract

Achieving high thermoelectric performance in n-type two dimensional (2D) semiconductors is imperative owing to the scarcity of efficient n-type thermoelectric materials compared to their p-type counterparts. In this context, we explored the synergistic effect of the conductive network coupled with optical phonon softening and avoided crossing phenomena, induced by Tellurium (Te) doping in HfSeS, resulting in a newly designed n-type quasi-ternary material (HfSeS_0.5Te_0.5) with a high thermoelectric figure of merit ZT ∼3.61 at a temperature of 1200 K. Our findings reveal that the presence of Hf–S antibonding near the Fermi level favours the formation of the conductive network and augment carrier effective mass, elicits an elevated Seebeck coefficient and leads to superior electrical transport properties in the quasi-ternary system. Additionally, the heavy dopant Te induces 60% softening of optical phonon frequency compared to parent materials. Consequently, we observe the occurrence of avoided crossing between acoustics and optical phonon branches, leading to a low lattice thermal conductivity κ_l ∼ 1.07 W m⁻¹ K⁻¹ at a temperature of 300 K. This study demonstrates the mechanisms such as conductive network, strong optical phonon softening and avoided crossing phenomena to achieve high thermoelectric performance significantly in the quasi-ternary material.